The present invention relates generally to the production of bent glass sheets, and, more particularly, to an improved method and apparatus for bending glass sheets to special configurations that incorporates an S-shaped transverse bend as well as a longitudinal bend.
Bent glass sheets are commonly used as glazing closures in vehicles such as automobiles and the like. For such applications, the glass sheets must be bent to precisely defined curvatures dictated by the configuration and outline of the vehicle openings in which the sheets are to be installed as well as the overall styling of the vehicle. At the same time, it is important that the bent sheets meet very stringent optical requirements so that the viewing area of the resulting shaped window is free of optical defects that would interfere with good vision through the window. For certain windows used as glazing closures, the bent glass sheets are tempered to strengthen them and increase their resistance to damage resulting from impact. Furthermore, in the less frequent breakage of tempered glass, the resulting particles are relatively small and smoothly surfaced and thus are less likely to cause damage to an occupant of a vehicle than untempered glass, which fractures under less severe loads and causes particles having jagged edges of a larger size that are more likely to injure vehicle occupants.
In recent years, the glass sheets have been press bent into complicated shapes while suspended from tongs. However, tongs tend to penetrate into the major surfaces of the glass and provide localized portions that have poor optical properties as well as providing mechanical weakness for the glass in the vicinity of the points gripped by the tongs.
It has also been customary in the glass sheet bending art to press bend the glass sheets supported in a horizontal position, either on a series of conveyor rolls, or by a gaseous hearth bed which supports the glass in close relation thereover by hot gas, lifting the glass from proximity to the support plane provided either by conveyor rolls or the gaseous support bed into engagement with a vacuum holder and then depositing the glass onto a ring-like member which is interposed between the plane of support and the bottom surface of the vacuum holder. Prior to the present invention, most of the shapes produced by such apparatus were either simple bends or compound bends comprising fairly simple curvatures in both mutually perpendicular directions. The complicated nature of the heating operation to prepare the glass to be soft enought to be shaped by press bending caused the leading edge of the glass to develop a higher temperature than the trailing edge of the glass. This further complicated the shaping of the glass to various complex shapes.
The reason for this complication was that the leading edge of the glass which developed a higher temperature during conveyance through the furnace was more likely to sag than the trailing edge of the glass which developed a lower elevated temperature as a result of conveyance through the furnace. Consequently, attempts to develop a reverse curvature transverse to the general longitudinal curvature to the sheets in a portion of the sheet near a furnace exit was not accomplished in the most efficient manner available.
Glass sheets have been bent to compound curvatures involving bends about mutually perpendicular axes by floating the glass sheets over gaseous support beds that change shape from a flat to a curved configuration transverse to the path of movement and also develop a downwardly convex shape in the direction of movement to impart a relatively simple shape to the glass. However, to the best of our knowledge, gas support beds of this type have not been used in combination with press bending molds comprising upper and lower molds of complementary shape that further shaped the glass sheets to the complicated shapes desired as will be brought out in a discussion of various patents that follows.
The shaping of glass sheets transversely of their path of movement while conveyed along a gaseous support bed that has a flat upstream portion that changes gradually to a desired transversely curved shape is shown in U.S. Pat. Nos. 3,223,501 to Fredley et al.; 3,291,590 to McMaster; 3,332,762 to McMaster et al.; 3,399,042 to McMaster et al; 3,409,422 to Gulotta; 3,497,340 to Dennison et al. and 3,526,489 to McPhail, for example. For example, McMaster et al. 3,399,042 skews the glass sheets conveyed over a gaseous support bed to shape the glass sheets about an axis extending obliquely of a straight side edge and Gulotta 3,409,422 develops a compound curve in glass sheets conveyed along a path over a gaseous hearth bed having a flat upper surface in its upstream zone, a final zone of compound curvature having convexly elevated curvatures extending both transversely and longitudinally of said path and an intermediate zone of a configuration changing progressively from a flat configuration to a configuration curved convexly transversely of said path. None of these patents shows a gas hearth bed that progressively changes from a flat configuration to a configuration curved convexly in elevation in the longitudinal direction and curved concavely in elevation in a direction transversely of a path taken by glass sheets over a gaseous hearth bed.
U.S. Pat. No. 3,846,104 to Seymour illustrates a number of patents that shape glass sheets to a non-uniform shape by delivering one or more heat-softened glass sheets at a time into a position at a shaping station over a gaseous hearth bed having an upwardly facing surface beneath an upper vacuum mold. A lower outline pressing mold of complementary shape is located in vertical alignment below the upper vacuum mold and moves between a retracted position below the upper surface of the bed and an upper position near the upper vacuum mold. The lower outline pressing mold rises to engage the glass sheet(s) against the upper vacuum mold to cause the heat-softened glass sheet to develop a shape conforming to that of the vacuum mold. Suction applied to the upper vacuum mold holds the shaped glass sheet against its lower shaping surface while the lower outline pressing mold restracts to enable a ring-like member having an outline shaping surface that conforms to the supported portion of the glass sheet adjacent its perimeter to shuttle into position below the vacuum mold. The vacuum on the vacuum mold is ended and the shaped glass sheet falls onto the ring-like member which supports the shaped glass sheet adjacent its perimeter for transfer to a cooling station where the glass is cooled sufficiently rapidly to develop a desired degree of temper. To the best of our knowledge, no apparatus of this type has been developed to shape glass sheets to a compound bend comprising a transverse bend component of S-shaped configuration and a longitudinal bend component of concave elevation.
U.S. Pat. No. 4,217,126 to Hagedorn et al. discloses press bending apparatus for bending glass sheets to compound curvatures including a reversely curved portion that is bent in a directin opposite the direction at which a glass sheet sags. The apparatus of this patent comprises a tunnel heating furnace, a shaping station having an upper shaping mold, a discontinuous ring-like lower shaping mold having a downwardly curved transversely extending shaping rail adjacent the furnace, shaping pads within the outline of the ring-like lower shaping mold and a series of spaced, special conveyor rolls that support a heat-softened glass sheet for movement into the shaping station where the lower shaping mold and shaping pads lift the glass sheet into engagement with the upper shaping mold and then retract to enable to special conveyor rolls to convey the bent glass to a cooling station. The lack of a continuous edge support for the shaped glass results in edge wrinkles. Also, the location of the downwardly curved transversely extending shaping rail adjacent the furnace complicates the shaping operation because this patent tries to impart a transverse bend to the trailing end portion of a glass sheet and the temperature gradient established along the glass sheet length comprises a more readily shapable hotter leading end portion and a less hot trailing end portion that is less likely to sag than the hotter leading end portion. Furthermore, since this patent redeposits the bent glass on spaced conveyor rolls before the bent glass is sufficiently cool to retain a permanent shape, the lack of a peripheral support during conveyance of the bent glass to the cooling station results in loss of control of glass sheet shape, particularly its reversely curved trailing end portion.
U.S. Pat. No. 4,265,650 to Reese et al. discloses apparatus for press bending glass sheets that are sag bent to an outline shape of an outline mold and are conveyed into a compound bending station having upper and lower press bending molds of complicated shape including a transverse bend of S-shaped configuration. There, the lower press bending mold lifts the preliminarily bent glass sheet into engagement with the upper press bending mold to develop a compound glass sheet shape. The lower press bending mold lowers to redeposit the compound bent glass onto the outline mold. The latter conveys the compound bent glass through an annealing lehr. This patent requires the glass to be overbent to shapes such that the subsequent sag during annealing compensates for the excess bending to develop the desired final shape. Also, a small margin of the glass remains outside the supporting area provided by the lower press bending mold. Accordingly, special hardware must be provided to force the unsupported glass edge against the press bending mold and avoid edge droop or edge wrinkles.
It would be beneficial for the glass sheet shaping art to develop a method and apparatus for bending glass sheets to a compound bend including a transverse bend component of S-shaped configuration and a longitudinal bend component that takes advantage of the temperature gradient established when the sheets move through a furnace en route to a shaping station and that maintains the complicated shape of the sheets throughout their entire marginal portion during their conveyance from a shaping station to a cooling station so that the compound bent glass sheets are free of edge droop or edge wrinkles and have major surfaces with acceptable optical properties.